1. Field
The described aspects relate to wireless communications, and more particularly, to transfer of a packet switched session between different types of communication systems.
2. Background
Evolved-Terrestrial Radio Access Network (E-UTRAN) is a packet switched (PS) only system and is not connected to a circuit switched (CS) domain. As such, no voice service may be provided in E-UTRAN. There have been many attempts to provide voice service for an E-UTRAN user equipment (UE) such as, for example, including Internet Protocol (IP) Multimedia Subsystem (IMS) with single radio voice call continuity (SFVCC), Circuit Switched (CS) fall back (CSFB), and Voice over Long Term Evolution (LTE) via Generic Access (VoLGA). Each of these methods assume that a UE is equipped with both E-UTRAN and Global System for Mobile communications (GSM) Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (GERAN) / Universal Terrestrial Radio Access Network (UTRAN) radio interfaces, yet activates only one of them at a time.
Some UEs may activate E-UTRAN and GERAN/UTRAN radio interfaces simultaneously. When a UE has two receivers and two transmitters and the UE is under both E-UTRAN and UTRAN/GERAN coverage, the UE registers and camps on UTRAN/GERAN for the CS domain while registering and camping on E-UTRAN for the PS domain. The UE may then use UTRAN/GERAN for CS services such as voice calls and SMS, and may use the E-UTRAN for PS services at the same time.
Problems may arise when a UE, as described above, moves away from the E-UTRAN coverage area. In this case, the UE's PS services and session are to move from E-UTRAN to UTRAN/GERAN. Typically, when the UE is in the connected mode in E-UTRAN, PS handover from the E-UTRAN to UTRAN/GERAN is performed to move the UE's PS sessions. However, the radio network controller (RNC) in UTRAN or the base station controller (BSC) in GERAN may not be equipped to handle a PS handover request for an UE that already has a connection for CS services.
Moreover, problems may arise when attempting to use the network-initiated PS handover when the UE is active in both UTRAN (for CS services) and E-UTRAN (for PS services), and when the UE is served by two RNCs in UTRAN, e.g. the two RNCs are cascaded (S-RNC and D-RNC) using the Iur interface to serve the UE. In this case, when the PS handover from E-UTRAN to UTRAN is performed, the target UTRAN enodeB is under the D-RNC, not the S-RNC. Because the E-UTRAN does not know that the UE is already active in the UTRAN and served by a different RNC, e.g. the S-RNC, than the RNC controlling the target UTRAN nodeB, e.g. the D-RNC, the E-UTRAN (e.g. the eNode B) triggers PS handover towards the D-RNC. This PS handover towards the D-RNC of the UE would fail and cause undefined network behavior, because in UTRAN only one RNC can act as a UE's S-RNC (serving RNC).
Thus, it would be desirable to improve the performance and behavior of inter-system handover.